Refinement of the nickel site structure in Desulfovibrio gigas hydrogenase using range-extended EXAFS spectroscopy.

We have reexamined the Ni EXAFS of oxidized, inactive (as-isolated) and H(2) reduced Desulfovibrio gigas hydrogenase. Better spatial resolution was achieved by analyzing the data over a 50% wider k-range than was previously available. A lower k(min) was obtained using the FEFF code for phase shifts and amplitudes. A higher k(max) was obtained by removing an interfering Cu signal from the raw spectra using multiple energy fluorescence detection. The larger k-range allowed us to better resolve the Ni-S bond lengths and to define more accurately the Ni-O and Ni-Fe bond lengths. We find that as-isolated, hydrogenase has two Ni-S bonds at approximately 2.2 A, but also 1-2 Ni-S bonds in the 2.35+/-0.05 A range. A Ni-O interaction is evident at 1.91 A. The as-isolated Ni-Fe distance cannot be unambiguously determined. Upon H(2) reduction, two short Ni-S bonds persist at approximately 2.2 A, but the remaining Ni-S bonds lengthen to 2.47+/-0.05 A. Good simulations are obtained with a Ni-Fe distance at 2.52 A, in agreement with crystal structures of the reduced enzyme. Although not evident in the crystal structures, an improvement in the fit is obtained by inclusion of one Ni-O interaction at 2.03 A. Implications of these distances for the spin-state of H(2) reduced H(2)ase are discussed.

Structure, physical, and photophysical properties of platinum(II) complexes containing bidentate aromatic and bis(diphenylphosphino)methane as ligands.

This study focuses on a series of PtII(L-L')(dppm)n+ complexes, where dppm is bis(diphenylphosphino)methane and L-L' are C-C' (n = 0), C-N (n = 1), and N-N' (n = 2) aromatic ligands. Structural characteristics are as follows: for [Pt(phen)(dppm)](PF6)2, a N-N' derivative, monoclinic, C2/c, a = 33.583(6) A, b = 11.399(2) A, c = 22.158(4) A, Z = 8; for [Pt(phq)(dppm)](PF6), a C-N derivative, triclinic, P1, a = 11.415(3) A, b = 13.450(3) A, c = 14.210(4) A, Z = 2; for [Pt(phpy)(dppm)](PF6), a C-N derivative, triclinic, P1, a = 10.030(3) A, b = 13.010(2) A, c = 15.066(4) A, Z = 2; and for [Pt(bph)(dppm)], a C-C' derivative, P2(1)/c, a = 17.116(7) A, b = 21.422(6) A, c = 26.528(6) A, Z = 12, where phen is 1,10-phenanthroline, phq is 2-phenylquinoline, phpy is 2-phenylpyridine, and bph is 2,2'-biphenyl. Structural features indicate that the Pt-C bond distance is shorter than the Pt-N bond distance in symmetrical complexes and that the Pt-P bond distance trans to N is shorter than the Pt-P bond trans to C. This is consistent with the 31P NMR spectra where the chemical shift of the P trans to C is approximately 10 ppm less than found for P trans to N. The energy maxima of the metal-to-ligand charge-transfer band for the complexes containing various L-L' ligands occur in the near-UV region of the spectrum and fall into the energy series bpy > bph > phen > 2-phpy > 2-ptpy > 2-phq > 7,8-bzq, where bpy is 2,2'-bipyridine, 2-phpy is 2-phenylpyridine, 2-ptpy is 2-p-tolylpyridine, and 7,8-bzq is 7,8-benzoquinoline. The emission energy maxima, ascribed to variance in metal-perturbed triplet ligand centered emission, commence near 500 nm and follow the series phen > bpy > 7,8-bzq > 2-phpy > 2-ptpy > bph > 2-phq. In general, emission is observed at 77 K and in solution at low temperatures, but the temperature dependence of the emission lifetimes indicates thermal activation to another state occurs with an energy of approximately 1800 cm-1 for the complexes, with the exception of [Pt(bph)(dppm)], which has an activation energy of approximately 2300 cm-1.